Help with LiFePo4

Hello All,

I am a total n00b so please bear with me. I have bought a Megawheels S1 electric scooter second hand. The charging socket was lose and broke after a few weeks. I replaced it and it worked for a while then it stopped taking any charge. I took out the battery and it appears to be a 7S1P LiFePo4 pack, with 25.2V nominal voltage. Interestingly the provided charger is 29.4V which begs the question: is it possible that I overcharged the battery? I pried open the battery pack and there is a BMS inside (see closeup on the right hand side photo) with RE2700000482 on it. The voltage of the seven cells, measured on the connectors of the BMS vary between 2.7V and 3.1V.
When I connect the charger, the status LED on the charger used to switch to red. It doesn't do that anymore. If I connect the same charger to a flat battery of an ebike then the status LED does change to red and it charges the battery as it should. I do not suppose the charger is faulty. I also experimented with another (29.4V/3A) to na avail: it charges the bicycle battery and does not charge this one.
Please advise. Do I need to replace the BMS? Or the battery pack?

I would suggest that it is necessary to change the battery. Variation in the cell at 0.4v for LiFePo4 is a sign of death. Most likely, the BMS does not charge because of this.
But judging by the photos, you have a li-ion battery. 4.2 v per cell * 7 = 29.4v
the battery pack is very bad = / sorry

I don't think the Pack is ''very bad'' yet.
It's a li-ion, not lifepo4 Battery

The cells that are sitting at under 3V are preventing the BMS from charging.
You could try charging the Cells with a 4.2V 1A (or less) charger through the BMS wires up to 3.1V or so and then fully charge the pack through the BMS.

29.4V is pretty much perfect full charge voltage for 8S LiFePo4, which is the normal pack configuration for 24V nominal. Are you sure it's a 7S LiFePo4 pack? I don't know if I've ever seen one before.

Edit: Just saw your last post, indicating it's not.

Watthour said:

7S1P LiFePo4 pack

Click to expand...

The BMS has a checkbox ticked on it for Li-Ion, rather than LiFePO4.

with 25.2V nominal voltage.

Click to expand...

25.2v / 7s = 3.6, also indicating Li-Ion rather than LiFePO4 (which would be 3.2v nominal per cell).

Interestingly the provided charger is 29.4V

Click to expand...

29.4v / 7s = 4.2v, which is the full voltage of Li-Ion.

The voltage of the seven cells, measured on the connectors of the BMS vary between 2.7V and 3.1V.

Click to expand...

2.7v is probably below the BMS's cutoff point for safe-to-recharge, so it is probably preventing charge to prevent a potential fire. Cells that go too low in voltage can be hazardous to recharge. Most likely yours are not that far gone, but it is always possible--there have been fires in a lot of situations that we cannot know the details of (because the pack was destroyed in the fire, and no detailed info from before the fire was taken down).

There's a number of reasons a cell could be that low compared to others, but usually it's that the cell is less capable than the others, so it cna't hold as much charge, and it can't handle as much current delivery.

There are a lot of battery pack repair threads here on ES that discuss how to recharge just one cell of a pack, with various methods ranging from safe but potentially more costly to rather hazardous but very cheap or even free, if you want to look them up and try to save the pack.

Some of the threads also discuss replacing the cells instead, either all of them or just the problematic ones.


Either way, it's probably not the BMS itself causing the problem.

It's possible, but unlikely, that the balancing resistor for the low cell is stuck on and draining it constnatly--but if that were the case, then it's likely that at the low voltage it's already at it would have drained it to nothing by the time you read this post, or at least to a significantly lower voltage than you last measured (while all other cells would remain the same).

The lower the cell voltage is, and the longer it stays there, the more likely it is to sustain permanent damage that makes it a fire risk.